Anodic oxidation methyl ester

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (—)camphoric anhydride or (S)-phenylalanine methyl ester . Much better results were obtained with the poly(L-valine) coated platinum electrodes . For example, f-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

Sulfides having two silyl groups are also oxidized electrochemically in methanol to give the corresponding methyl esters (Scheme 11 [36, 37]. The alkylation of (phenylthio)bis(trimethylsilyl)methane with alkyl halides followed by the anodic oxidation provides a convenient access to esters with one-carbon... 

The mechanism shown in Scheme 23 has been suggested The first step involves the transfer of an electron from the acylsilane to produce the cation-radical intermediate. Attack of methanol at the silicon cleaves the C-Si bond to give the acyl radical intermediate, although there is no direct evidence for the acyl radical intermediate. The acyl radical is then oxidized anodically to the acyl cation, which reacts with methanol to give the corresponding methyl ester. 

Various nucleophiles other than methanol can be introduced onto the carbonyl carbon. Anodic oxidation of acylsilanes in the presence of allyl alcohol, 2-methyl-2-propanol, water, and methyl /V-methylcarbamate in dichlorometh-ane affords the corresponding esters, carboxylic acid, and amide derivatives (Scheme 24) [16]. Therefore, anodic oxidation provides a useful method for the synthesis of esters and amides under neutral conditions. 

Low asymmetric induction (e.e. 0.3-2.5%) was found (56) to occur when unsymmetrical sulfides were electrochemically oxidized on an anode modified by treatment with (->camphoric anhydride or (5)-phenylalanine methyl ester. 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

Steckhan and coworkers found that the indirect anodic oxidation of N-pro-tected dipeptide esters 56, in which the C-terminal amino acid is a-branched, can afford methyl imidazolidin-4-one-2-carboxylate 57 in 45-84% yields [86], This reaction can be performed at a Pt-anode by using Et4NCl as an electrolyte in the presence of 5% methanol in an undivided cell (Scheme 30). 

Anodic oxidation of halogenated tyrosines was studied in connection with some sponge metabolites (cavemicolin model compounds). The methyl exter of 3,5-dibromotyrosine afforded four different products in a 41 10 26 23 ratio with 23% overall yield as a result of equilibration. (Scheme 44) [93JCS(P2)3117], A related compound was obtained as a mixture of stereoisomers 56 from a Diels-Alder reaction between N-acetyldehydroalanine methyl ester and l-methoxy-l,3-cyclohexadiene (87TL2371). 

The electrochemical behavior of malonyl-a-aminopyridines 661 was investigated by Gullu et al. in acetonitrile or a mixture of trifluoroacetic acid and dichloromethane containing tetrabutylammonium tetrafluoro-borate or triethylammonium trifluoroacetate in a water-jacketed, two-compartment glass cell equipped with a platinum disk anode at 1.50 V (Ag/ Ag+) and a carbon-rod secondary electrode (91T675). Controlled potential anodic oxidation of 661 afforded labile coupled carboxylic acids 662 (R2 = COOH), which easily decarboxylated to compounds 662 (R2 = H) under the work-up conditions. Sometimes, the carboxylic acid 662 (R2 = COOH) could be isolated or when the reaction mixture was treated with methanol, methyl ester 662 (R = H, R1 = Bu, R2 = COOMe) was obtained in 40% yield. 

ANODIC OXIDATION OF N-CARBOICTHOXYPYRROLIDINE 2-HETHOXY-N-CARBOtCTHOXYPYRROL ID INE (1-Pyrrolidinecarboxy11c acid, 2-Methoxy-, Methyl ester)... 

Anodic oxidation in methanol of thiophene and substituted thiophenes at low temperatures (—20 to — 30°C) results in methoxylation and ring opening with loss of sulfur as SO2 [190]. From thiophene is isolated butenedialdehyde tetramethylacetal, some methoxysuc-cinic dialdehyde tetramethylacetal, and a small amount of methyl y6-formylpropionate. In general, oxidation of thiophenes results in the formation of derivatives of a, -unsaturated y-dicarbonyl compounds or y-keto esters. Cyanomethoxylation of 2,5-dimethyl thiophene yields mainly cis- and rm/i5-2-cyano-5-methoxy-2,5-dimethyldihydrothiophene and 3-cyano-2,5-dimethylthiophene [191]. 

The anodic oxidation in methanol of sulifides and esters having two silyl groups leads to the formation of the corresponding methyl esters [Eq. (31)]. Since (phenylthio)-bis(trimethylsilyl)methane and methoxybis(trimethylsilyl)methane can be easily deproto-nated and alkylated, these compounds serve as effective synthons of the anion of the methoxycarbonyl group [118,119,122]. 

Esters 34 have been used as substrates in Kolbe electrolysis reactions. Anodic oxidative coupling of the sodium salt of 34b gives (25, 55)-diethyl 2,5-dihydroxyadipate (37) in 54% yield with > 95% ee [37]. Electrolysis of 34a in the presence of pentanoic acid with a stabilized current (60 V, 1.5— 2 A) over a period of 50 h produces methyl (5)-2-acetoxyheptanoate (38) in 48% yield [36]. Electrolysis of 34a with an excess of methyl dimethylmalonate in methanol containing sodium methoxide affords the (5)-acetoxydiester 39 [38]. 

In an undivided cell the yields are reduced substantially. Methyl hexanoate gave (co — 1) and also some (co — 2) products. The selectivity of the reaction is reminiscent of the ester chlorination with N-chloro amines . In the anodic oxidation, however, carbocations seem to be involved, being formed most remote from the ester function. In suitable cases carbocation rearrangements are observed. 

---